The present invention relates to development control in elecrorostatographic imaging and, more particularly, to liquid toner development control.
Generally, there are two types of development systems employed by electrostatographic imaging apparatus, namely, powder toner development systems and liquid toner development systems. Although powder toner is more conventional, liquid toner is often preferred for its higher intrinsic resolution. Considerable efforts have been made in the past to design more efficient and more convenient liquid toner development systems.
Liquid toner systems are sensitive to physical changes in the toner, such as changes in temperature, charge level, viscosity and liquid concentration, most of which are not relevant in powder toner systems. It is appreciated that these toner changes may affect the development level, thereby resulting in inconsistent imaging. Therefore, control of the liquid toner properties is generally considered to be crucial for maintaining a constant level of developed mass per unit area (DMA) on a photoreceptor of the imaging apparatus.
One current approach to maintaining image quality measures the optical density, volume and conductivity of the liquid toner used in the process. Based on these measurements, toner concentrate, carrier liquid a charge directs, respectively are added to the liquid toner. Such an approach is described in U.S. Pat. No. 4,860,924, the disclosure of which is incorporated herein by reference.
It is appreciated that construction and maintenance of a closed loop development system as described above is both complex and expensive. Therefore, liquid toner development systems have never been embodied in low-cost disposable cartridges, as normally is the case in powder toner systems.
In U.S. Pat. No. 4,341,461, the bias voltage of a development rolls in a powder development system la adjusted in accordance with a measurement of toner density on a developed patch on a photoreceptor. The toner density is measured by an infrared densitometer which apparently measures the optical density of the layer of toner developed on the photoreceptor.
U.S. Pat No. 4,678,317 describes a liquid toner system in which a sensor electrode is used to sense the potential of a charged photoreceptor and to adjust a development electrode voltage to compensate for variations in the sensed potential
WO 93/01531, the disclosure of which is incorporated herein by reference, describes a direct-transfer liquid toner development system. A layer of concentrated liquid toner coating a toning roller is brought into virtual contact with a photoreceptor, and portions of substantially even thickness are transferred from the toning roller onto attractive portions the photoreceptor. Either the full thickness of the portions is transferred, is a binary mode of operation or, in a quasi-binary mode of operation, a partial yet even thickness is transferred. The voltage between the toning roller and the photoreceptor determines the thickness of the layer which is transferred. In the binary mode, the DMA on the photoreceptor is substantially equal to the DMA on the toning roller and, in the quasi-binary mode, the photoreceptor DMA is dependent in a well defined manner upon the toning roller DMA. For quasi-binary transfer the photoreceptor DMA is generally more uniform than the toning roller DMA.
The direct-transfer system described above normally employs a toner applicator and a squeegee associated with the toning roller.
It is so object of the present invention to provide an improved liquid toning system. In accordance with a preferred embodiment of the present invention, consistent toning of latent electrostatic images is maintained throughout numerous toning cycles without adding liquid toner or liquid toner components to the system and/or adjusting the material composition of the liquid toner, i.e. the ratio between toner particles and carrier liquid.
In general, liquid toner including charged toner particles and carrier liquid is contained in a sump of the toning system. The toner particles are selectively removed from the liquid toner during the toning process as they are transferred to a latent image bearing surface such as a photoreceptor. The carrier liquid is generally removed at a different rate, usually a lower rate. Thus, the percentage of toner particles in the liquid toner, hereinafter referred to as the solids concentration, rises or falls as a function of the total area toned by the toning system. For some colors, for which the proportion of printed surface to unprinted surface is small, the solids concentration may rise with time.
When either the solids concentration or the total quantity of liquid toner in the system is reduced below a pre-set value, either the sump or the entire toning system is replaced or refilled.
In accordance with a preferred embodiment of the present invention, there is thus provided a direct transfer toning system inducting an endless toning surface, preferably the surface of a toning roller charged to a predetermined voltage, coated with a lays of tone concentrate, a developed mass pet unit area (DMA) controller having an input for receiving an indication of the DMA on an imaging outface such as a photoreceptor, and adjusting the DMA on the toning surface in response to the received input, whereby the DMA on the toning roller is maintained substantially constant.
Preferably, the DMA controller controls at least one voltage which affects the DMA on the toning roller.
According to one aspect of the present invention, the input to the DMA controller is supplied by a DMA sensor which monitors the DMA on the imaging surface. Since, in direct-transfer toning systems, the DMA on the imaging surface is dependent upon the DMA on the toning rolls, by controlling the DMA on the toning roller, a consistent toning level is readily maintained.
In one embodiment of this aspect of the invention, the DMA sense includes an optical sensor which monitors the optical density (OD) on the surface of the photoreceptor or, alternatively, on the surface of the toning roller and supplies an indication of the OD to the input. In this case, the DMA controller includes a comparator which compares the signal to a value representative of a desired DMA and adjusts at least one voltage to product the desired DMA.
In accordance with another aspect of the present invention, the input to the DMA controller is generated by a solids concentration indicator responsive to the solids concentration of the liquid toner. In this aspect of the invention the development system preferably further includes apparatus for measuring the temperature of the toner. Based on the solids concentration indication and the measured toner temperature, the at least one voltage is adjusted according to a look-up table to provide the desired DMA.
According to one, preferred, embodiment of this aspect of the invention, the solids concentration indicator includes a concentration detector which measures the concentration of solids in the toner. The concentration detector may include a viscosity sensor an optical sensor, a permitivity sensor or a sensor of any other property of the tone which is related to the solids concentration.
According to another, preferred, embodiment of this aspect of the invention, the solids concentration indicator includes a concentration calculator which generates an output responsive to the total area toned by the toning system since the last refill/replacement of the toning system. Since the total toned area can be approximated by the number of toning cycles performed by the system, the concentration calculator may include a counter of the number of toning cycles performed since the last refill/replacement of the system it is appreciated that the concentration of solids is the liquid toner is substantially a function of the total area towed and, thus, only approximately, a function of the number of toning cycles performed by the system
Alternatively or additionally, the proportion of printed to none-printed area on each of the cycles is calculated and the amount of carrier liquid and toner particles per page is determined. In this embodiment the concentration calculation would be improved over the concentration calculation of the previous embodiment.
In a preferred embodiment of the invention, the concentration calculator is at least partially comprised in a xe2x80x9csmart chipxe2x80x9d which is part of the cartridge. In this case, the smart chip stores specific concentration information for the cartridge. This allows replacement of cartridges without having to reset any counts on the computer. For example, it is sometimes useful to print with inks having special properties, such as fluorescent inks or non-process color inks. Since these cartridges are used only intermittently and must be removed when another special color is to be printed, it is very useful to have the concentration information attached to the cartridge itself.
The accuracy of the calculation of toner particle usage may be improved by using the DMA measurement to determine more accurately the amount of toner particles per unit printed area. A level detector in the sump may be used to determine the amount of liquid toner which has been removed from the sump. This determination, together with the determination of the amount of toner particles used in printing, can be used to give a very accurate determination of the concentration.
For improved development control, the liquid toner in the development system preferably includes a toner charge stabilizer operative for maintaining a substantially constant level of electric charge per unit mass (hereinafter Q/M) in the liquid toner. In a preferred embodiment, the toner charge stabilizer includes a charge director.
Further, in accordance with a preferred embodiment of the invention, the development system includes an applicator manifold for supplying liquid tone and coating the toning surface with a layer of concentrated liquid toner. A portion of the applicator manifold juxtaposed with the toning surface, hereinafter referred to as the coating electrode, is preferably charged to a relatively high voltage which aids the coating process. Preferably, the DMA controller includes apparatus for adjusting the voltage on the applicator manifold.
Preferably, the toning system includes a squeegee roller associated with the toning surface and electrified to a voltage different from that of the toning surface. Preferably, the DMA controller controls the squeegee voltage on the squeegee roller is response to the input received from the DMA monitor or the concentration indicator and the temperature sensor, in accordance with the alternative aspects of the present invention described above.
For the preferred embodiment described herein, the DMA on the toning surface is a function, inter alia of the voltages on the applicator manifold and the squeegee rolls.
In a preferred embodiment of the invention, the squeegee roller is urged against the surface of the toning roller by the action of a leaf spring. The portion of the leaf spring in contact with the squeegee roller is preferably coated with a compressible pad which is, more preferably, formed of a closed cell foam or elastomer.
In a preferred embodiment of the present invention, the toning system is embodied in a replaceable cartridge.